As a result of recent changes in emissions regulations, there is widespread interest in reducing engine speeds in modern vehicles. In particular, it is desirable to reduce the idling speed of vehicle engines as far as possible, and also the engine speed at which automatic transmission systems select a higher gear, in order to comply with environmental legislation.
At the same time, the demand for electrical power in vehicles has been rising. This is in part due to the fact that modern vehicles typically include a large range of internal systems such as climate control, seat warming, infotainment systems, and so on. Often, the time when the electrical demand is highest coincides with a period of engine idling or low engine speed. For example, when a vehicle is first started and for a short time subsequently, the internal systems all initiate while the engine idles. Some systems, in particular heating, ventilation and air-conditioning (HVAC) systems, run at high power during this time in order to prepare the driving environment.
Although it is possible to use some stored battery power to meet the electrical demand, it is generally preferred to avoid depleting battery resources during normal running of the vehicle, to conserve battery power for start-up operations and avoid premature aging of the battery. It is therefore desirable to match the electrical generating capacity of the vehicle to the electrical load at all times.
Conventionally, electrical power is generated by an engine-driven alternator, and the generating capacity of the alternator is directly related to the speed at which the alternator is driven. Typically, an armature of the alternator is driven directly by the engine using an auxiliary drive belt. The drive belt is coupled to the engine and the armature using respective pulleys. The ratio of the pulleys is typically configured to provide a threefold increase in alternator speed relative to the engine speed.
Accordingly, in this arrangement the alternator speed is directly proportional to the engine speed. Therefore, if the pulley ratio is selected so as to provide sufficient generating capacity when the engine is idling, the alternator speed may be very high when the engine is running near a maximum speed. For example, if the armature must be rotated at 3000 rpm to serve the maximum electrical requirement of the vehicle, a 1000 rpm engine idling speed would satisfy this requirement with a 3:1 pulley ratio. Therefore, when the engine speed is near a maximum, typically 7000-9000 rpm, the alternator is spun at approximately 21000-27000 rpm. This is much faster than is necessary to satisfy the electrical power demand, which is wasteful. Furthermore, such high speed operation increases wear and potentially reduces the average time to component failure, and also necessitates robust bearings, which are expensive.
It is therefore desirable to reduce the maximum speed at which the alternator is driven, whilst ensuring the generating capacity remains sufficient during engine idling. One way to increase the generating capacity of the alternator is to increase the size of the armature, or even to add a second armature. These approaches allow the pulley ratio to be reduced, and therefore reduce the maximum speed of the alternator, while maintaining the required generating capabilities. However, this approach is not favoured, since raising the size of the armature goes against a general objective of reducing vehicle weight, in line with the overriding purpose of reducing emissions. Furthermore, in the case of a larger, heavier armature, more robust bearings would be required. For an arrangement including either a larger armature or multiple armatures, parasitic losses would increase due to increased friction.
An alternative approach has been proposed in which a mechanical gearbox is provided between the alternator and its respective pulley. In this arrangement, the gear has two selectable ratios, with an appropriate ratio being selected with respect to the instantaneous engine speed. A higher ratio can be selected at lower engine speeds to maximise generating capacity, and a lower ratio can be selected at higher engine speeds to reduce the drive speed of the alternator. Since two distinct gear ratios are provided, a drawback with this arrangement is that there is a changeover time whilst moving between the gears during which the alternator is not driven. Thus, during this time, generation drops significantly. A further consideration is that the mechanical gearbox introduces extra cost, weight, vibration/noise, complexity and failure potential, and there is currently a desire within the automotive industry to exchange mechanical systems for electrical systems where possible.
Another alternative approach has been proposed in which a variable speed magnetic gear, such as is described in US 2011/0037333 A1, is used in the place of a mechanical gearbox. A known variable speed magnetic gear includes three concentric and coaxial rotors. The innermost and outermost rotors each have a respective set of permanent magnets disposed evenly around their circumference. An intermediate rotor, which is disposed between the inner and outer rotors, comprises a set of core members. The core members are arranged to interact with the magnetic field between the two sets of magnets, such that movement of the intermediate member induces movement of the inner rotor. When the outer rotor is stationary, a fixed gear ratio is defined between the inner rotor and the intermediate rotor. Accordingly, an input shaft may be coupled to the intermediate rotor, and an output shaft may be coupled to the inner rotor, so as to provide a fixed speed increase between the input and the output shafts. In order to vary the gear ratio as required, so as to provide a variable speed increase, the outer rotor is physically rotated. This modulates or influences the coupling between the intermediate and inner rotors. An external means, for example a motor, must be provided for driving the outer rotor.
The combination of the magnetic gear with the external means for driving the outer rotor is complex, large and heavy. Therefore, this approach is unlikely to be preferable to simply increasing the size of the armature of the alternator.
Against this background, it would be desirable to provide a gear for use in an improved vehicle generating system, which overcomes or at least substantially alleviates the disadvantages known in the prior art.